Funding agencies: This study was supported by grants from the National Program of Basic Research (2010CB945200, 2011CB504104) of China, National Natural Science Fund (81071024, 81171202, 30872729, 30870879 and 81129018), Shanghai Shuguang Program (11SG20), and the Fifth National Undergraduate Student Innovating Program (2011015) and Public Benefit Research Foundation from Ministry of Health (201202008).

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